This invention relates to investment casting operations and, more specifically, the invention relates to a method and means for supporting cores and the like during formation of such castings. The supporting function is particularly intended to guard against core movement or shifting which can occur in the various stages of the operation.
Investment casting procedures are frequently employed for the production of castings having one or more internal passages. Turbine blades and vanes comprise examples of cast articles defining hollow interiors which function to provide cooling for the blades or vanes during use.
In order to provide the internal passages, it is necessary to use cores which are usually of ceramic composition. Typically, the cores have "prints" which extend beyond the pattern portion defining the wall of the article to be cast so that these "prints" will be embedded in the ceramic material employed for forming the casting mold. When the metal is introduced into the mold cavity, the supported ends will tend to prevent displacement of the core which would result in improper location for the passage to be formed. For example, if a core is bent when encountering molten metal being introduced into a mold, the thickness of the wall which separates the casting exterior from the internal passage may become intolerably small.
As the performance requirements for turbine blades and vanes have increased, the cooling requirements, and thus the type of passages formed in such articles, have become more complex. The result is that the support for cores provided by the surrounding mold has been found to be inadequate since even small deviations of the core from its preferred position can lead to reject parts. In addition, it has been found that the deviation of cores from a desired location can occur during pattern removal, during curing of ceramic molds, and at elevated temperature preheating of ceramic molds.
Core displacement during casting is more likely where castings are formed as single crystals or by processes involving directional solidification. In those cases, there is a more gentle introduction of molten metal, but the mold containing the core is at elevated temperature when the metal is poured, and the mold is kept in this condition for a long period of time. The disturbance of the core position during pattern removal and mold curing is, of course, also a factor.
Various attempts have been made to provide means for supporting cores independently of the support provided by a mold. Chaplets such as described in Gibson U.S. Pat. No. 2,096,697 represent well-known prior art core supporting techniques. Other techniques specifically developed for use in connection with ceramic molds are set forth in Bishop U.S. Pat. No. 3,596,703 and Rose U.S. Pat. No. 3,659,645. It will be clear from a review of this prior art, however, that the primary concern involves the disturbance of core position as the metal is being poured. Core displacement during pattern removal, during mold curing and during mold preheating is not discussed.
Such prior systems have also failed to deal with the problems associated with the positive metal left on the casting surfaces by chaplet prints in the mold. These problems include but are not limited to finishing, dimensional control, inclusion control, nucleation and recrystalization. Specifically, the prior arrangements have utilized chaplets and the like which extended into the ceramic material of the mold, and the space occupied by such material was filled with cast material as the chaplet or other support dissolved in the course of the casting operation. This left protuberances on the cast surface which had to be removed by a finishing operation.